FIG. 4 shows a capacitor structure in a conventional semiconductor device, wherein numeral 30 denotes a semiconductor substrate, numeral 31 denotes an insulating film, for example, made of film oxide, and numeral 32 denotes an electrode film formed of a polysilicon film, the electrode film 32 functioning as one electrode of the capacitor while the semiconductor substrate 30 as the other electrode thereof.
However, in order to obtain a capacitor having a large capacitance with a small capacitor area in a structure shown in FIG. 4, the insulating film 31 needs to be formed of a material having a relatively high dielectric constant and to be made thin. As an insulating film which is easily formed on a semiconductor substrate, the film made of silicon oxide, silicon nitride or the like is employed. However, the dielectric constant of such a film is not very high, for example, 4 to 8. Moreover, such an insulating film is usually relatively thick, or several hundred .ANG., and the capacitance of a capacitor having such film is relatively small, for example, about 5 FF/.mu.m.sup.2. Accordingly, a larger capacitor area is needed for obtaining a larger capacitance, and thus the capacitor occupies a larger area in a chip. This arises a problem of going against the demand for higher integration and the recent inclination of small-sized devices.